The evolution of the \(Z_{\mathcal {N}}\)  axion field
and its contribution to the DM relic abundance
departs drastically from the standard case [1]}.
The cosmological impact of hypothetical parallel “mirror” worlds has been studied at length in the literature (for a review, see e.g. Ref. [2]}).
Crucially, the constraints on the number of effective relativistic species \(N_{\text{eff}}\)  imply that the mirror copies of the SM must be less populatedMechanisms that source this world-asymmetric initial temperatures while preserving the \(Z_{\mathcal {N}}\)  symmetry may arise naturally in the cosmological
evolution [3]}, [4]}. –cooler– than the ordinary SM world.
<FIGURE>